The present invention is directed to a method for releasing mercury.
Methods and systems for releasing mercury are used particularly in fluorescent lamps.
The method of dosing directly liquid mercury by means of syringe feeders is unable to provide an exact and reproducible dosage of the smaller and smaller amounts of the element which are required by the present lamps.
Some known methods are based on mechanical systems being loaded with metallic mercury. For example, U.S. Pat. Nos. 4,823,047 and 4,278,908 disclose capsules, made of metal or glass, respectively, containing liquid mercury, while U.S. Pat. No. 4,808,136 and European Patent Application Publication No. EP 568,317 disclose the use of porous pills or spherules (made of metallic or ceramic material, respectively), being impregnated with mercury which is then released by heating. However, also with these methods, the released amount of mercury is hardly reproducible and, mainly in the case of capsules, constructional problems may arise.
Other documents disclose the use of mercury compounds, such as U.S. Pat. No. 3,657,589 relating to Ti—Zr—Hg compounds (of particular importance being the compound Ti3Hg) or U.S. Pat. No. 5,520,560 dealing with the use of compounds according to U.S. Pat. No. 3,657,589 in admixture with copper-tin alloys having functions of promoting the mercury release. However, these compounds require rather high temperatures for the mercury releasing, generally in excess of 500° C., whereby a specific high temperature thermal process is required in order to produce metallic mercury within the sealed lamp.
Finally, there is a great number of documents relating to amalgams being employed, such as International Patent Publication No. WO 94/18692 pertaining to amalgams with zinc or U.S. Pat. No. 5,598,069 pertaining to amalgams with indium-silver. However, the amalgams generally have a mercury content being not particularly important and above all they have a tendency to release mercury already at relatively low temperatures, e.g., about 100° C. The amalgams can thus lose amounts of mercury which are not negligible even during lamp manufacturing steps, which is undesirable, with possible pollution of the working environment. For example, the lamps may undergo heat treatments to enhance the removal of gaseous impurities being trapped in the phosphors without being yet cooled down to room temperature when the amalgam is introduced, thus starting to release mercury when the lamp is not yet sealed.